1. Field of the Invention
The present invention relates to a light emitting diode, and more particularly, to a light emitting diode with an improved laminated structure of a P-type semiconductor layer for more smoothly injecting holes into an active layer.
2. Discussion of the Background
In general, since Group III element nitrides, such as GaN, AlN and InGaN, have excellent thermal stability and a direct transition type energy band structure, they have recently come into the spotlight as materials for light emitting diodes (LEDs) in blue and ultraviolet regions. Particularly, an InGaN compound semiconductor has been noticed for its narrow band gap. LEDs using such a GaN-based compound semiconductor are used in various applications such as large-sized full-color flat panel displays, backlight sources, traffic lights, indoor illumination, high density light sources, high resolution output systems and optical communications.
High-frequency white LEDs are currently expected to replace fluorescent lamps. In particular, efficiency of white LEDs has reached the level similar to that of typical fluorescent lamps. However, efficiency of LEDs can continue to improve. Particularly, increasing internal quantum efficiency by improving crystal quality and a structure of an epitaxial layer is required.
An LED generally has a structure in which an active layer is interposed between an N-type semiconductor layer and a P-type semiconductor layer. Electrons and holes are injected into the active layer from the N-type and P-type semiconductor layers, respectively, and the electrons and holes are recombined in the active layer, thereby emitting light.
A variety of trials are being conducted to increase the recombination rate of electrons and holes by optimizing the N-type semiconductor layer, the P-type semiconductor layer, and the active layer. In addition, increased internal quantum efficiency may be achieved by improving a structure of the semiconductor layers.
Meanwhile, impurities from the P-type semiconductor layer are diffused into the active layer, and therefore, the active layer may deteriorate. The impurities diffused into the active layer form electron traps, thereby lowering luminous efficiency. Therefore, preventing unintended impurities from diffusing into the active layer is desired.
In the meantime, the layers are generally formed by a metal organic chemical vapor deposition (MOCVD) technique. In order to form epitaxial layers with excellent crystal quality, the layers are generally formed in-situ. In the in-situ process, after each layer is formed, a process for changing a source gas is performed. At this time, gas containing nitrogen, e.g., ammonia (NH3) is introduced into a chamber without any reactive gas in order to prevent a deposited epitaxial layer from decomposing.
However, the ammonia reacts with the deposited epitaxial layer and decomposes. Then, hydrogen contained in the ammonia is combined with a P-type impurity, e.g., Mg to prevent Mg from being activated in the semiconductor layer. Combinations of the hydrogen and Mg near a surface of the laminated structure may decrease to a certain extent by a heat treatment process. However, it is difficult to decompose the combinations of the hydrogen and Mg inside the laminated structure. Thus, the production and mobility of holes is reduced, thereby lowering internal quantum efficiency.
Therefore, an improved laminated structure of a P-type semiconductor layer, which can prevent hydrogen and doped impurities from being combined, is required.